1. Field of the Invention
The present invention relates to methods of analytical sample preparation for instrumental analysis. More specifically, the present invention relates to a capillary microextraction technique for preconcentrating trace analytes.
2. Description of Related Art
Sample preparation is an important step in chemical analysis, especially when dealing with traces of target analytes dispersed in complex matrices. Such matrices are commonplace in samples from environmental, petrochemical, and biological origins. Samples of this nature are not generally suitable for direct introduction into analytical instruments. Such incompatibility is related to two factors. First, the complex matrices may have a detrimental effect on the performance of the analytical system or they may interfere with the analysis of the target analytes. Second, the concentration of the target analytes in the sample may be so low that it goes beyond the detection limit of the analytical instrument. In both cases sample preparation is necessary to make the sample compatible with analytical instrumentation. This is achieved through sample clean-up and sample preconcentration. Sample derivatization is also sometimes necessary to facilitate analysis and detection of target compounds.
Sample preparation in chemical analysis often involves various extraction techniques to isolate and preconcentrate target compounds from complex matrices in which they exist in trace concentrations. Conventional extraction techniques (e.g., liquidxe2x80x94liquid extraction (Majors, R. E. LC*GC. Int. 1997, 10, 93-101), Soxhlet extraction (Lopez-Avila, V.; Bauer, K.; Milanes, J.; Beckert W. F. J. AOAC Int. 1993, 76, 864 880), etc. frequently used for this purpose are often time-consuming and involve the use of large volumes of hazardous organic solvents.
To address environmental and health concerns associated with the use of large volumes of organic solvents and to reduce sample preparation time, newer extraction techniques have been developed that use either reduced amounts of organic solutes such as solid-phase extraction (SPE), (Coulibaly, K; Jeon I. J. Food Rev. Int. 1996, 12, 131-151), accelerated solvent extraction (ASE) (Richter, B. E.; Jones, B. A.; Ezzel, J. L.; Porter N. L.; Abdalovic N.; Pohl, C. Anal. Chem. 1996, 1033-1039), microwave-assisted solvent extraction (MASE) (Zlotorzynski, A. Crit. Rev. Anal. Chem. 1995, 25, 43-76), etc. Another approach to address these problems was to develop sample preparation techniques using alternative, less hazardous extraction media, such as supercritical fluid extraction (SFE) (Hawthorne, S. B.; Anal. Chem. 1990, 62, 633A-642A). However, the extraction technique which is most fascinating from the environmental and occupational health and safety points of view is solid-phase microextraction (SPME) developed by Pawliszyn and coworkers (Berladi, R. P.; Pawliszyn, J. Water Pollut. Res. J. Can. 1989, 24, 179-91; Arthur, C. L.; Pawliszyn, J. Anal. Chem. 1990, 62, 2145). SPME completely eliminates the use of organic solvents for the extraction of analytes from a wide range of matrices. Another important feature of SPME is that, unlike conventional extraction techniques, it does not require exhaustive extraction-establishment of equilibria between the sample matrix and the stationary phase coating is sufficient to obtain quantitative extraction data. For most samples, the equilibration time is less than 30 minutes, which places SPME among the fastest extraction techniques.
In SPME, the outer surface of a solid fused silica fiber (approximately 1 cm at one end) is coated with a selective stationary phase. Thermally stable polymeric materials that allow fast solute diffusion are commonly used as stationary phases. The extraction operation is carried out by simply dipping the coated fiber into the sample matrix and allowing time for the partition equilibrium to be established. The sensitivity of the method, is mostly governed by the partition coefficient of an analyte between the coating and the matrix. Extraction selectivity can be achieved by using appropriate types of stationary phases that exhibit high affinity toward the target analytes.
In its traditional format, SPME has a number of drawbacks. First, since the stationary phase coating is applied to the outer surface of the fiber, it is more vulnerable to mechanical damage. Second, traditional methods for the preparation of coatings fail to provide adequate thermal and solvent stability to the thick stationary phase (several tens of micrometers in thickness) coatings that are needed in SPME. This is due to the lack of chemical bonding between the coatings and the substrate to which they are applied.
In recent years, the extraction of analytes by GC stationary phase coatings on the capillary inner surface has received considerable attention. The introduction of in-tube SPME had the primary purpose of coupling SPME to high-performance liquid chromatography (HPLC) for automated applications. The in-tube SPME method uses a flow-through process where a coated capillary is employed for the direct extraction of the analytes from the aqueous sample. The extraction process involves agitation by sample flow in and out of the extraction capillary. Successful coupling of in-tube SPME with HPLC, as well as HPLC-MS, has been achieved for the specification of organoarsenic compounds, (Wu, J.; Mester, Z.; Pawliszyn, J. Anal. Chem. 1999, 71, 4237-4244) and determination of rantidine, (Kataoka, H.; Lord, H. L.; Pawliszyn, J. J. Chromatogr. B 1999, 731, 353-359) xcex2-blockers, (Kataoka, H.; Narimatsu, S.; Lord, H.; Pawliszyn, J. Anal. Chem. 1999, 71, 4237-4244) carbamate pesticides, (Gou, Y.; Pawliszyn, J. Anal. Chem. 2000, 72 2774-2779; Gou, Y.; Eisert, R.; Pawliszyn, J. J. Chromatogr. A 2000, 873, 137-147; Gou, Y.; Tragas, C.; Lord, H.; Pawliszyn, J. J. Micro Sep. 2000, 12, 125-134) and aromatic compounds (Wu, J.; Pawliszyn, J. J. Chromatogr. A 2001, 909, 37-52).
In spite of rapid on-going developments, especially in the areas of in-tube SPME applications, a number of fundamental problems remain to be solved. First, GC capillaries that are used for in-tube SPME typically have thin coatings that significantly limit the sample capacity (and hence sensitivity) of the technique. Conventional static coating techniques (Bouche, J.; Verzele, M. J. Gas Chromatogr. 1968, 6, 501-505; Janak, K.; Kahle, V.; Tesarik, K.; Horka, M. J. High Resolut. Chromatogr./Chromatogr. Commun. 1985, 8, 843-847; Sumpter, S. R.; Woolley, C. L.; Hunag, E. C.; Markides, K. E.; Lee, M. L. J. Chromatogr. 1990, 517, 503-519) used to prepare stationary phase coatings in GC columns are designed primarily for creating thin (sub-micrometer thickness) coatings. Thus, developing an alternative technique to provide higher coating thickness suitable for in-tube SPME applications is very important. Second, usually the stationary phase coatings used in GC capillaries are not chemically bonded to the capillary surface. In conventional approaches, these relatively thin coatings are immobilized on the capillary inner surface though free-radical cross-linking reactions. (Wright, B. W.; Peaden, P. A.; Lee, M. L.; Stark, T. J. J. Chromatogr. 1982, 248, 17-34; Blomberg, L. G. J. Microcol. Sep. 1990, 2, 62-68) Immobilization of thicker coatings (especially the polar ones) is difficult to achieve. (Janak, K.; Horka, M.; Krejci, J. J. Microcol. Sep. 1991, 3, 115-120; Berezkin, V. G.; Shiryaeva, V. E.; Popova, T. P. Zh. Analit. Khim. 1992, 47, 825-831) Third, because of the absence of direct chemical bonding between the stationary phase coating and the GC capillary inner walls, the thermal and solvent stabilities of such coatings are typically poor or moderate. When such extraction devices are coupled to GC, reduced thermal stability of thick GC coatings leads to incomplete sample desorption and sample carryover problems. (Buchholz, K. D.; Pawlyszyn, J. Anal. Chem. 1994, 66, 160-167; Zhang, Z.; Yang, M. J.; Pawliszyn, J. Anal Chem. 1994, 66, 844A-853A.; Potter, D. W.; Pawliszyn, J. Environ. Sci. Technol. 1994, 28, 298-305)
Low solvent stability of conventionally prepared thick stationary phase coatings present a significant obstacle to the hyphenation of in-tube SPME with liquid-phase separation techniques that employ organic or organo-aqueous mobile phase systems of the desorption of analytes. Solvent stability of the in-tube SPME coatings is, therefore, fundamentally important for further development of the technique. (Wu, J.; Pawliszyn, J. Anal Chem. 2001, 73, 55-63) Thus, these three problems need to be solved in order to exploit full analytical potential of in-tube SPME.
In accordance with the present invention, there is provided a method of preconcentrating trace analytes by extracting polar and non-polar analytes through a sol-gel coating and/or sol-gel monolithic bed. The present invention further provides a microextraction method including the steps of micro-extracting polar and non-polar analytes in a sol-gel coating and/or sol-gel monolithic bed, desorbing the analytes from the sol-gel and analyzing the extracted, desorbed analytes.